In order to detect the direction of defocus and the amount thereof, there is known such an arrangement that light of a photographic object is passed through a photographic lens and to form two light images of the object, which are directed two regions existing symmetrical with respect to the light axis of the lens, thereby reforming two light images on two planes, so that a displacement and a direction between the positions of the two focused light images from a predetermined focused position are detected for determining when the photographic lens is in the in-focus position, front focus position or rear focus position. FIGS. 1A and 1B show an example of the arrangement for the method described above, wherein behind the photographic lens 2, a condenser lens 4 is placed on the predetermined focal plane 4 or backward therefrom. Re-forming lenses 8 and 10 are placed behind the condenser lens 4 and first and second photo line sensor arrays 14 and 16 are disposed. By this arrangement, in the case of a front focus condition, light images projected on the photo sensor arrays 14 and 16 come near the optical axis and in case of a rear focus condition, the light images are displaced away from the optical axis. In case of an in-focus condition, the two light images are projected on predetermined positions defined by the optical system of the focus sensing device. Therefore, by detecting the distance between the two light images on the photo sensor arrays, the focused condition of the photographic lens can be detected.
In FIGS. 2(a) through 2(c), light image patterns projected on the first and second photo sensor arrays 14 and 16 are shown. The first photo sensor array 14 (referred to as the standard photo sensor array hereinafter) consists of photo cells L1 through L9 and the second photo sensor array 16 (referred to as the reference photo sensor array hereinafter) consists of photo cells R1 through R21 lined up in a horizontal direction with a predetermine space S on both sides of a center line 18. The center line 18 is determined so that the optical axis of the photographic lens 2 passes through it. Patterns of the light intensity of the images projected on the standard and reference photo sensor arrays 14 and 16 are designated by LIm and RIm. FIG. 2(a) shows that the image pattern RIml which coincides with the image pattern LIm1 projected on the standard photo sensor array 14 is positioned on photo cells R7 through R15 of a reference photo sensor array 16. It is assumed that FIG. 2(a) shows an in-focus condition. In case of FIG. 2(a), the output value of each photo cells L1 through L9 of the standard photo sensor array 14 is equal to the output value of each photo cells R7 through R15 of the reference photo sensor array 16 respectively. Therefore L1-R7=0, L2-R8=0, . . . L9-OR15=0, wherein L1, L2, . . . L9, R7, R8, . . . R15 are output values of the respective photo cells L1 through L7 and R7 through R15. In addition, .vertline.L1-R7.vertline.+.vertline.L2-R8.vertline.+. . . .vertline.L9-R15.vertline.=0. As mentioned above, in a case where two images coincide with each other, the difference between the corresponding two photo cells is 0 and the sum of each difference is also 0. In practice, since the photo sensing characteristics of the respective photo cells are different from cell to cell, the result of the subtraction and the total sum can not be zero but can be a minimum if the degree of the coincidence between two images is highest.
In case of a front focus condition, as shown in FIG. 2(b), the image pattern RIm2 which is projected on the reference photo sensor array 16 and coincides with the image pattern LIm2 projected on the standard photo sensor 14 is projected on the photo cells R1 through R9. In case of a rear focus condition, as shown in FIG. 2(c), the image pattern RIm3 which is projected on the reference photo sensor array 16 and coincides with the image pattern LIm3 projected on the standard photo sensor 14 is projected on the photo cells R13 through R21. In order to detect the projected position of the image pattern RIm on the reference photo sensor array 16, the image pattern LIm on the photo cells L1 through L9 of the standard photo sensor array 14 is compared with the image pattern RIm on the photo cells R1 through R9 of the reference photo sensor array 16, thereafter the image pattern LIm on the photo cells L1 through L9 is compared with the image pattern on the photo cells R2 through R10. In a similar manner, the image pattern is taken from the respective photo cells of the reference photo sensor array 16 by shifting the measurement to the right by one photo cell, then the respective image patterns thus taken are compared with the image pattern LIm so that results of comparison of thirteen sets can be obtained. Defining H(1) as the result of the comparison in terms of the image pattern on the photo cells R1 through R9, the result can be expressed. ##EQU1##
In a similar manner, the result H(2) of the comparisons in terms of the photo cells R2 through R10 is expressed ##EQU2##
By the calculation mentioned above, thirteen results of the comparisons H(i), wherein i=1,2, . . . ,13 (referred to as relative value hereinafter) can be obtained.
By finding the minimum relative value Hmin(n) among H(1) through H(13), the number n represents the result of the comparison wherein both image patterns LImn and RImn coincides best. For example, in case of FIG. 2(a), the minimum relative value occurs at the number n=7. In case of FIGS. 2(b) and 2(c), the minimum relative values occurs at the numbers n=1 and n=13. The amount of the displacement of FIGS. 2(b) and 2(c) against FIG. 2(a) is -6(=1-7), 6(=13-7) on the pitch of the photo cells basis, then the amount of the displacement can be obtained.
By providing 9 photo cells of the standard photo sensor array 14 and 21 photo cells of the reference photo sensor array 16, the defocus condition can be detected in the range of a plus or minus 6 pitches. In order to expand the detectable range of defocus, it is possible to increase the number of the photo cells of the reference photo sensor array 16 against the number of the photo cells of the standard photo sensor array 14. In order to expand the detectable range of the defocus with the 21 photo cells of the reference photo sensor array, it is required to decrease the number of the photo cells of the standard photo sensor array 14. For example, detection of the defocus can be made with 7 pitches by eliminating the photo cells L1 and L9. Decreasing the number of the photo cells, however, causes the accuracy of the defocus detection to be lowered.
In order to eliminate the disadvantage described above, U.S patent application Ser. No. 540,012 discloses a method of detecting the defocus with a high accuracy as shown in FIG. 3. In FIG. 3, there are added four photo cells La, Lb, Lc and Ld to the standard photo sensor array 14 shown in FIG. 1. Using the output of 13 photo cells La through Ld in addition to the photo cells L1 through L9, the defocus can be detected within the range of +4 pitches of the photo cells. Compared with the arrangement of FIG. 2, the defocus detecting range of the arrangement shown in FIG. 3 is smaller by +2 pitches of the photo cells, thereby the accuracy of the comparison can be improved.
In order to detect the defocus condition with greater than 4 pitches of the photo cells, defocus as shown in FIG. 3(b) can be detected using seven photo cells L5 through Ld. FIG. 3(b) shows that the image pattern is displaced by 10 pitches of the photo cells relative to the in-focus condition shown in FIG. 3(a). In other word, by using the output of the photo cells L5 through Ld of the arrangement shown in FIG. 3, a front focus condition defocused by 10 pitches of the photo cells can be detected. To the contrary, using the output of the photo cells La through L5 enables the system to detect the rear focused condition defocused backwardly by 10 pitches. In the arrangement of FIG. 3, the accuracy of the defocus detection may be decreased due to decrement of the number of the photo cells. However, this may be negligible because in case of a large defocus, it is enough to detect a rough value of the defocus first, then the objective lens is moved to a position near the in-focus position on the basis of the detected defocus value, subsequently a fine focusing detection is made using the output of the photo cells La through Ld.
In the focus detection, at first, the position of the photographic lens is not preliminarily known, therefore one problem is to decide what photo cell outputs among the group I, II, III shown in FIG. 3 should be used at first as the standard. In the U.S. patent application Ser. No. 570,012, defocus detection is made first for all of the groups I, II and III, then the minimum relative value Lmin(n) is detected so as to select any one of the groups I,II, III in which the minimum relative value occurs. In the above method, the defocus detection for the groups I,II and III must be made even if the photographic lens is situated near the focused position, therefore such an operation is apparently unnecessary.